(329g) Hydrogen Direct Reduced Ironmaking: Technoeconomics of Titanomagnetite Ironsand Reduction in a High-Temperature Fluidised Bed

Problem:

The manufacture of iron and steel is responsible for approximately 7% of global man-made carbon dioxide emissions. [1] The use of hydrogen as a reducing agent can substantially reduce carbon dioxide emissions from the ironmaking process when hydrogen is derived from renewable resources such as renewable electricity. Our work has shown that pellets of hydrogen direct reduced iron (H2-DRI) can be produced in a vertical shaft furnace, however the profitability is highly dependent on the cost of electricity. [2] The capital and operating costs associated with a H2-DRI process can be significantly reduced if the iron ore pelletising and sintering steps are eliminated.

Iron ore fines can be reduced relatively quickly when compared to pellet reduction in a shaft furnace. While experiments with iron ore fines in a fluidised bed using hydrogen at high temperatures (> 800 °C) have been carried out, several studies have reported sticking of the particles. [3-5] Agglomeration of the sticking particles caused the bed to defluidise, effectively shutting down the process.

Methods:

Here, we report results from the reduction of New Zealand (NZ) titanomagnetite (TTM) ironsands in a small-scale (100 g of ironsand) laboratory fluidised bed reactor at temperatures between 800 – 1000 °C using hydrogen flow rates up to 5 standard L/min (Figure 1). Hydrogen enters the vertical quartz tube and fluidises the ironsand after passing through a quartz frit inside the tube.

Iron oxide reduction in hydrogen is carried out through several stages as follows:

3 Fe₂O₃ + H₂ → 2 Fe₃O₄ + H₂O (1)

2 Fe₃O₄ + 2 H₂ → 6 FeO + 2 H₂O (2)

6 FeO + 6 H₂ → 6 Fe + 6 H₂O (3)

Overall Reaction: 3 Fe₂O₃ + 9 H₂ → 6 Fe + 9 H₂O (4)

By considering the thermodynamic equilibrium between hydrogen, steam, and the oxidation state of iron of each step, the extent of ore reduction was tracked via a humidity sensor on the outlet of the reactor. Ironsand samples (~2 g) were extracted throughout the reduction run via a venturi sampling nozzle for analysis.

Extracted samples were characterized by powder X-ray diffraction [7] to determine which phases were present and the metallisation degree. Individual grains were analysed via scanning electron microscopy (SEM) and elemental energy-dispersive X-ray spectroscopy (EDS).

Results:

No sticking phenomena were observed, and a metallization degree of ~93% was achieved in 30 minutes using pure hydrogen at 950 °C. Sticking was inhibited by the formation of a protective Ti- and Al-rich outer oxide layer (Figure 2 and Figure 3) on the majority of particles, which prevented iron-iron contact.

To determine the economic viability of the process, we estimated the operating energy expenditures for the H2-DRI process carried out at 900 °C. As a first approximation we modelled each step of the process (Figure 4) through steady-state mass and energy balances. The original analysis was carried out assuming a counter-current vertical shaft reactor. However, the counter-current reactor can be approximated by a cascading series of fluidized bed reactors as described, for example, by POSCO’s HyREX® process. [8]

The largest energy consumption in the process was associated with production of hydrogen (Figure 5) via electrolysis. Due to reaction equilibrium limitations, the reactor exit contains significant unreacted hydrogen (e.g. 46% hydrogen at 900°C). It is therefore essential to condense the steam and separate water from the unreacted hydrogen and recycle the hydrogen back to the reactor to continue to drive the reduction reactions and maintain economic viability.

Moreover, the cost of renewable electricity has a significant impact on the economic feasibility of the H2-DRI process. Figure 6 indicates that an input renewable electricity cost (at the H2-DRI production facility) of well below US$80/MWh is required if H2-DRI is to compete on a cost basis against existing carbothermic DRI processes. Due to the continuous nature and scale of a commercial H2-DRI process, it is important that electricity is sourced from firm (continuously operating) electricity generating facilities.

Implications:

The findings are important as increasing the operating temperature of the fluidised bed reactor beyond 800°C results in a faster reaction rate and higher gas utilisation, making the process more economically attractive. This has the potential to reduce the size of the reduction reactors and lower the capital costs of a H2-DRI facility in a final commercial-scale process. Moreover, obtaining long-term low-cost pricing contracts from firm renewable electricity generating assets (e.g. nuclear, geothermal, or hydroelectric) is required to ensure a profitable and therefore economically and environmentally sustainable process.

References:

1. https://ourworldindata.org/ghg-emissions-by-sector

2. Cassidy van Vuuren, Ao Zhang, James T. Hinkley, Chris W. Bumby, Matthew J. Watson, The potential for hydrogen ironmaking in New Zealand, Cleaner Chemical Engineering, Volume 4, 2022, 100075, ISSN 2772-7823, https://doi.org/10.1016/j.clce.2022.100075.

3. Zhan Du, Yu Ge, Fan Liu, Chuanlin Fan, Feng Pan, Effect of different modification methods on fluidized bed hydrogen reduction of cohesive iron ore fines, Powder Technology, Volume 400, 2022, 117226, ISSN 0032-5910, https://doi.org/10.1016/j.powtec.2022.117226.

4. Wolfinger T, Spreitzer D, Schenk J. Analysis of the Usability of Iron Ore Ultra-Fines for Hydrogen-Based Fluidized Bed Direct Reduction—A Review. Materials. 2022; 15(7):2687. https://doi.org/10.3390/ma15072687

5. Qiyan Xu, Haichuan Wang, Yuankun Fu, Jianjun Wang, Parameters Optimization, Sticking Mechanism and Kinetics Analysis of Fine Iron Ore in Fluidized-Reduction Process, ISIJ International, 2016, Volume 56, Issue 11, Pages 1929-1937, Released on J-STAGE November 17, 2016, Advance online publication September 13, 2016, Online ISSN 1347-5460, Print ISSN 0915-1559, https://doi.org/10.2355/isijinternational.ISIJINT-2016-280

6. Sigit W. Prabowo, Raymond J. Longbottom, Brian J. Monaghan, Diego del Puerto, Martin J. Ryan, Chris W. Bumby, Phase transformations during fluidized bed reduction of New Zealand titanomagnetite ironsand in hydrogen gas, Powder Technology, Volume 398, 2022, 117032, ISSN 0032-5910, https://doi.org/10.1016/j.powtec.2021.117032.

7. Prabowo, S.W., Longbottom, R.J., Monaghan, B.J. et al. Sticking-Free Reduction of Titanomagnetite Ironsand in a Fluidized Bed Reactor. Metall Mater Trans B, 50, 1729–1744 (2019). https://doi.org/10.1007/s11663-019-01625-w

8. https://www.posco.co.kr/homepage/docs/eng7/jsp/hyrex/

Figure Captions:

Figure 1: Small-scale fluidised bed reactor.[6]

Figure 2: SEM images showing the microstructural evolution of grains of TTM ironsand during the reduction by 100% hydrogen at 950 °C: (a) raw ironsand, (b) 5 min, (c) 10 min, (d) 20 min, (e) 40 min, and (f) 60 min. [7]

Figure 3: EDS images showing elemental maps of a grain of TTM ironsand reduced in 100% hydrogen at 950 °C for 60 min. (a) is the BSE image, and (b) to (f) refer to the element noted in each image. [7]

Figure 4: Process flow diagram of hydrogen direct reduction of iron. [2]

Figure 5: Energy requirements for the hydrogen DRI process.[2]

Figure 6: Production cost for H2-DRI as a function of electricity cost. [2] Orange wedge represents the available processing margin for H2-DRI. The yellow region represents the additional available margin for billet steel from H2-DRI.